1. Field of the Invention
The invention relates generally to roller cone drill bits for drilling earth formations, and more specifically to roller cone drill bits having optimized cutting element counts for reduced tracking and/or increased drilling performance.
2. Background Art
Roller cone rock bits are commonly used in the oil and gas industry for drilling wells. FIG. 1 shows one example of a roller cone drill bit used in a conventional drilling system for drilling a well bore in an earth formation. The drilling system includes a drilling rig 10 used to turn a drill string 12 which extends downward into a well bore 14. Connected to the end of the drill string 12 is roller cone drill bit 20.
A roller cone drill bit typically includes a bit body with a threaded connection at one end for connecting to a drill string and a plurality of roller cones, typically three, attached at the other end and able to rotate with respect to the bit body. Disposed on each of the cones is a plurality of cutting elements, typically arranged in rows, about the surface of the cones. The cutting elements may comprise tungsten carbide inserts, polycrystalline diamond compacts, or milled steel teeth.
Significant expense is involved in the design and manufacture of drill bits to produce drill bits with increased drilling efficiency and longevity. Roller cone bits are more complex in design than fixed cutter bits, in that the cutting surfaces of the bit are disposed on roller cones. Each of the roller cones independently rotates relative to the rotation of the bit body about an axis oblique to the axis of the bit body. Because the roller cones rotate independent of each other, the rotational speed of each cone is typically different. For a given cone, the cone rotation speed generally can be determined from the rotational speed of the bit and the effective radius of the “drive row” of the cone. The effective radius of a cone is generally related to the radial extent of the cutting elements on the cone that extend axially the farthest, with respect to the bit axis, toward the bottomhole. These cutting elements typically carry higher loads and may be considered as generally located on a so-called “drive row”. The cutting elements located on the cone to drill the full diameter of the bit are referred above to as the “gage row”.
Adding to the complexity of roller cone bit designs, cutting elements disposed on the cones of the roller cone bit deform the earth formation by a combination of compressive fracturing and shearing. Additionally, most modern roller cone bit designs have cutting elements arranged on each cone so that cutting elements on adjacent cones intermesh between the adjacent cones, as indicated for example at 29 in FIG. 2 and further described in U.S. Pat. No. 5,372,210 to Harrell. Intermeshing cutting elements on roller cone drill bits is typically desired to minimize bit balling between adjacent concentric rows of cutting elements on a cone and/or to permit higher insert protrusion to achieve competitive rates of penetration (“ROP”) while preserving the longevity of the bit. However, intermeshing cutting elements on roller cone bits substantially constrains cutting element layout on the bit, thereby, further complicating me designing of roller cone drill bits.
Because of the complexity of roller cone bit designs, roller cone bits have been largely developed through a trial and error process that involves selecting an initial design, field testing the initial design, and then modifying the design to improve drilling performance. For example, when a bit design has been shown to result in cutting elements an one cone being worn down faster than cutting elements on another cone, a new bit design may be developed by simply adding more cutting elements to the cone that bad cutting elements that wore down faster in hopes of reducing wear on each of the cutting elements on that cone.
In more recent years, this trial and error process has been used in conjunction with other processes and programs proposed to predict characteristics associated with the drilling performance of the bit. For example, U.S. Pat. Nos. 6,213,225 and 6,986,395, issued to Chen, propose an optimization process for equalizing the downward (axial) force on each of the cones of a drill bit. U.S. Pat. Nos. 6,516,293 and 6,873,947, issued to Huang et al., disclose methods for designing roller cone drill bits which include simulating the drilling performance of a bit, adjusting a design parameter, and repeating the simulating and adjusting until an optimized performance is obtained.
The problem with current roller cone drill bit designs is that the resulting arrangements ore often arrived at somewhat arbitrarily. As a result, many prior art bits may provide less than optimal drilling performance due to problems which may not be readily detected, such as “tracking” and “slipping.” Tracking occurs when cutting elements on a drill bit fall into previous impressions formed by other cutting elements at preceding moments in time during revolution of the drill bit. Slipping is related to tracking and occurs when cutting elements strike a portion of previous impressions made and then slide into the previous impressions rather than cutting into the uncut formation.
Cutting elements do not cut effectively when they fall or slide into previous impressions made by other cutting elements. In particular, tracking is inefficient because no fresh rock is cut. Slipping also should be avoided because it can result in uneven wear on cutting elements which can result in premature cutting element failure. Thus, tracking and slipping during drilling can lead to low penetration rates and in many cases uneven wear on the cutting elements and cone shell. By making proper adjustments to the arrangement of cutting elements on a bit, problems such as tracking and slipping can be significantly reduced. This is especially true for cutting elements on a drive row of a cone because the drive row generally governs the rotation speed of the cone.
Prior art exists for varying the orientation of asymmetric cutting elements on a bit to address tracking concerns. For example, U.S. Pat. No. 6,401,839, issued to Chen, discloses varying the orientation of the crests of chisel-type cutting elements within a row, or between overlapping rows of different cones, to reduce tracking problems and improve drilling performance. U.S. Pat. Nos. 6,527,068 and 6,827,161, issued to Singh, disclose methods for designing bits by simulating drilling with a bit to determine its drilling performance and then adjusting the orientation of at least one non-axisymmetric cutting element on the bit and repeating the simulating and determining until a performance parameter is determined to be at an optimum value. U.S. Pat. No. 6,942,045, issued to Dennis, discloses a method of using cutting elements with different geometries on a row of a bit to cut the same track of formation and help reduce tracking problems. However, in many drilling applications, such as the drilling of harder formations, the use of asymmetric cutting elements such as chisel-type cutting elements are not desired due to their poorer performance in these applications.
Prior art also exists for using different pitch patterns on a given row to address the tracking concerns. For example, U.S. patent application Ser. No. 10/853,869 (now U.S. Pat. No. 7,234,549) and Ser. No. 10/854,067 (now U.S. Pat. No. 7,292,967), titled “Methods for evaluating cutting arrangements for drill bits and their application to roller cone drill bit designs,” which are assigned to the assignee of the present invention and incorporated herein by reference, disclose, inter alia, designing drill bits by varying the pitch pattern between cutting elements in a row to help reduce tracking problems and improve drilling performance.
While the above approaches are considered useful in particular applications, in other applications the use of asymmetric cutting elements is not desired and the use of different pitch patterns can be difficult to implement and can result in a more complex approach to drill bit design and manufacture than necessary for addressing tracking concerns. What is desired is a simplified design approach that results in reduced tracking for particular applications without sacrificing bit life or requiring increased time or cost associated with design and manufacturing.